1. Field of the Invention
The present invention relates generally to the field of optical imaging, and more specifically to optical systems used for microscopic imaging, inspection, metrology and lithography applications.
2. Description of the Related Art
Many optical systems have the ability to inspect or image features on the surface of a specimen, such as inspecting defects on a semiconductor wafer or photomask, or alternately examining a biological specimen on a slide. Microscopes have been used in various imaging situations, including biology, metrology, semiconductor inspection, and other complex inspection applications where high resolution images of small areas and/or features are desired.
When inspecting features on a specimen surface, a broadband objective exhibiting highly accurate and near perfect optical performance is particularly desirable. This is particularly true for alignment systems such as those used for photomask alignment. However, all standard objectives are typically limited at the edge of the field by lateral color.
In general, lateral color represents a difference between different wavelengths of light, such as blue light and red light. FIG. 1 shows the principal ray of an optical system formed from a positive lens 101. Blue light ray 102 in this simple arrangement tends to be more strongly refracted than red light ray 103 due to the refractive index changing with wavelength. The difference in position of the rays is the lateral color 104, a standard definition based on the principal ray position. Note that the standard definition for lateral color does not include the effects of monochromatic aberrations such as coma and chromatic variation of coma. These aberrations can move the image centroid away from the principal ray location. Monochromatic aberrations and the chromatic variation of the monochromatic aberrations produce an additional contribution to lateral color when based on such an image centroid definition.
Lateral color according to this centroid definition can yield problems when inspecting under precise conditions. In other words, lateral color causes a shift in the image centroid for different colors. The centroid shift can adversely impact the image, particularly at the edge of the field. Lateral color can also limit the accuracy obtained when using optics in metrology applications.
Some new methods for high order color correction can reduce the effects of lateral color and the aforementioned chromatic centroid shift to less than 1 nanometer (nm) over a very broad wavelength range, generally improving the image at the edge of the field. As centroid shift can vary for different colors, a design where the centroid shift for all colors is reasonably uniform can be highly desirable, particularly when changing the focus position for the image.
It would therefore be beneficial to provide a system for use in microscopy that overcomes the foregoing lateral color drawbacks present in previously known systems and provide an optical inspection or metrology system design having improved functionality over devices exhibiting those negative aspects described herein.